Spectral Induced Polarization Research Articles

Spectral induced polarization measurements to detect different scales in oil and gas production tubing

Different varieties of scales, such as iron sulfide (FeS) scale, and calcium carbonate (CaCO3), commonly precipitate within oil and gas production tubing, posing operational challenges and production losses. It is essential to implement efficient mitigation strategies and preserve production integrity to detect and characterize these scales in an accurate and efficient manner. Prior studies have explored Spectral induced Polarization (SIP) for the early detection of FeS within porous media, mainly for environmental applications or for identifying FeS in carbonate formations from the perspective of reservoir characterization. This study focuses on applying SIP to address production-related issues in production tubing. For this purpose, six actual subsurface scale samples were analyzed using X-ray Diffraction, X-ray Fluorescence, and SIP techniques. These samples were categorized into three primary groups based on their composition: Pyrrhotite, Goethite, and Calcium Sulfate Hydrate or Calcite. The results indicate that combining phase shift and conductivity as frequency functions makes it possible to develop an approach to classify and differentiate between various scale types. This information could be instrumental in developing methods that can be potentially integrated into tubing systems to detect and characterize the different scale types.

Spectral induced polarization of corrosion of sulfur modified Iron in sediments

Spectral induced polarization (SIP) responses are not well understood within the context of remediation applications at contaminated sites. Systematic SIP studies are needed to gain further insights into the complex electrical response of dynamic, biogeochemical states to enable the use of SIP for subsurface site characterization and remediation monitoring. Although SIP measurements on zero valent iron have been previously published, the SIP response for sulfur modified iron (SMI), a similar potential subsurface reductive amendment, has not yet been reported. Hence, the purpose of this laboratory-scale study was to evaluate SIP for nonintrusive monitoring of SMI under relevant subsurface conditions. SMI was separately mixed with silica sand or sediments from the Hanford Site (Washington, USA) and then packed into columns for geochemical and SIP analysis for up to 77 days under fully saturated conditions. SMI exhibited distinguishable phase peaks between 0.1 and 1.0 Hz, which changed in magnitude based on content and were detected as low as 0.3 wt%. In the initial days, the complex conductivity, phase maxima, and chargeability increased while the peak locations shifted to higher frequency (decreasing relaxation times), suggesting an initial increase in polarization and concurrent decrease in the length scales (potentially due to changes in particle size and mineralogy). Then, after 77 days, the phase maxima and chargeability decreased with a concurrent increase in relaxation times, suggesting that over longer periods, less polarizable phases are forming and particle size or connectivity of polarizable phases is increasing. These results demonstrated a unique SIP response to SMI transformations that might be applied to monitoring of SMI emplaced as a subsurface barrier or injected in the field.

Influence of dissolved and non-aqueous phase toluene on spectral induced polarization signatures of soils

Fifty-two laboratory experiments are undertaken to analyze the sensitivity of spectral induced polarization (SIP) to the presence of toluene in soils. Among these experiments, four experiments are conducted to collect SIP responses of soils containing dissolved phase toluene within the pore water using columns. The results demonstrate that SIP is not sensitive to the presence of dissolved phase toluene in soils. The remaining forty-eight experiments are undertaken with four types of soils mixed with non-aqueous phase toluene. The experimental results prove that SIP is sensitive to toluene saturation under varying salinity conditions. These observations are well-explained by a published petrophysical model accounting for the effects of water saturation on complex conductivity. The water saturation exponent n and quadrature conductivity exponent p in this model are obtained by fitting complex conductivity data versus saturation at different saturation levels. The petrophysical model is tested where in-phase and quadrature conductivity responses are predicted from water saturation, soil cation exchange capacity (CEC), and pore water conductivity. The petrophysical model provides satisfactory predictions for non-aqueous phase toluene saturation. Overall, this study contributes to our understanding of SIP as a non-intrusive tool for characterizing toluene contamination in soils with applications to the field.

Spectral induced polarization (SIP) measurements across a PFAS-contaminated source zone

There is a need to develop field-scale, in situ screening technologies for assessing variations in aqueous film-forming foam (AFFF) concentrations in soils at former fire training and storage sites. Field-scale Spectral Induced Polarization (SIP) geophysical measurements were acquired on a transect crossing an AFFF source zone. Soil samples were acquired to determine variations in poly- and per-fluoroalkyl substances (PFAS) concentrations in soils, characterize soil texture, and create triplicate soil columns for laboratory SIP measurements. Field and laboratory observations show that SIP measurements are sensitive to the concentration of AFFF constituents associated with soil pore surface area. The specific polarizability and the phase of the SIP measurements for the laboratory samples were linearly correlated with total soil-sorbed PFAS concentration. The phase from the field SIP measurements was highest over the location of maximum PFAS concentration measured on the laboratory samples. However, a significant correlation between field-measured phase and laboratory-measured total PFAS concentration still needs to be established. These observations, along with the demonstrated sensitivity of the SIP response to the removal of soil PFAS using a methanol wash procedure, support the case for SIP characterization of AFFF source zones.

Study on the Dispersion Characteristics of Coal Complex Resistivity under the Effect of Liquid Nitrogen Cyclic Freeze-Thaw and Evaluation of Permeability Enhancement.

The effect of liquid nitrogen freeze-thaw fracturing on coal seams can be potentially evaluated by the complex resistivity method. The real part (Reρ) and the imaginary part (Imρ) of the complex resistivity and permeability of coal were determined under different cycle times and in different bedding directions. The reason for permeability enhancement was discussed, and the dispersion mechanism of complex resistivity during cyclic freeze-thaw fracturing was analyzed. The results indicated that (1) the complex resistivity parameters have a good response to the cycle times; Reρ, |Imρ|, and the dispersion degree (α) are positively correlated with cycle time; the fully polarized frequency (f p) of Reρ, the characteristic frequency (f c) of Imρ, and variation are negatively correlated with cycle time. (2) The difference in complex resistivity parameters between the vertical bedding direction and the parallel bedding direction is significant, and the difference in electrical properties of the bedding structure continuously decreases with the increase in cycle time. (3) Under the effect of liquid nitrogen cyclic freeze-thaw, a complex network of fractures in coal is formed, the anisotropic characteristics of coal are weakened, and effective conductive channels are damaged. The peak frost heave force decreases exponentially with the increase in cycle time, and the difference in bedding electrical properties gradually disappears. (4) Comparing the inversion degree of measured data with three conductive models, ρ0 and τ are selected as the optimum parameters for evaluating the effect of liquid nitrogen cyclic freeze-thaw. A logarithmic permeability evaluation model is constructed based on ρ0 and τ. This work provides a new perspective based on electrical detection for evaluating the permeability enhancement of coal during liquid nitrogen cyclic freeze-thaw.

Low Resistivity Pay Zone Detection in Hydrocarbon Formation: The Feasibility of the Spectral Induced Polarization Method

Summary Identifying and characterizing low resistivity pay (LRP) zones within hydrocarbon-rich formations has long been challenging in the petroleum industry due to their complex mineral composition, microporosity, and diminished resistivity contrasts. Traditional methods, such as resistivity measurements, struggle to effectively pinpoint LRP zones, prompting the need for innovative approaches in reservoir evaluation. This paper explores the feasibility of using the spectral induced polarization (SIP) method for detecting LRP zones. The SIP method measures complex conductivity across a frequency range from 1 mHz to 10 kHz. While this technique has been widely used in mining and environmental studies, its potential for petrophysics applications in the oil and gas sector remains largely unexplored. This study acts as a proof of concept, demonstrating the capability of SIP for detecting LRP zones. Laboratory experiments utilized dual-porosity silica gel samples with controlled micro- and macroporosity fractions and added pyrite content. Despite a high crude oil saturation of approximately 60%, the presence of brine in continuous micropores resulted in low resistivity readings (0.7 Ω·m) at low frequencies, as conventionally measured by direct current resistivity tools. However, at higher frequencies (>100 Hz), the study observed high average resistivity values (82 Ω·m), indicating a frequency-dependent behavior in electrical measurements. This behavior is attributed to polarization mechanisms, including the electrical double layer (EDL). This study’s findings propose the SIP method’s potential effectiveness for detecting LRP zones, paving the way for future research to delve deeper into the application of SIP in petrophysics.

Spectral induced polarization tomography inversion: Hybridizing homotopic continuation with Bayesian inversion

Induced polarization tomography offers the potential to better characterize the subsurface structures by considering spectral content from data acquisition over a broad frequency range. Spectral induced polarization (SIP) tomography is generally defined as a nonlinear inverse problem commonly solved through deterministic gradient-based methods. To this end, the spectral parameters, i.e., direct current resistivity, chargeability, relaxation time, and frequency exponent, are resolved by individually or simultaneously inverting all frequency data, followed by fitting a generalized Cole-Cole model to the inverted complex resistivities. Due to the high correlation between the Cole-Cole model parameters and a lack of knowledge about the initial approximation of the spectral parameters, using the classical least-squares methods may lead to inaccurate solutions and impede reliable uncertainty analysis. To cope with these limitations, we introduce a new approach based on a hybrid application of a globally convergent homotopic continuation method and a Bayesian inference to reconstruct the distribution of the subsurface spectral parameters. The homotopic optimization, owing to its fast and global convergence, is first implemented to invert multifrequency SIP data sets aimed at retrieving the complex-valued resistivity. Then, Bayesian inversion based on a Markov chain Monte Carlo (MCMC) sampling method, along with a priori information including the lower and upper bounds of the prior distributions, is used to invert the complex resistivity for the Cole-Cole model parameters. By applying the MCMC inversion algorithm, a full nonlinear uncertainty appraisal can be provided. We numerically evaluate the performance of our method using synthetic and real data examples in the presence of topographical effects. Numerical results prove that the homotopic continuation method outperforms the classic, smooth inversion algorithm in the sense of approximation accuracy and computational efficiency. In addition, we determine that our hybrid inversion strategy provides reliable representations of the main features and structure of the earth’s subsurface in terms of the spectral parameters.

Microwave complex conductivity measurement of superconducting very thin FeSe1−xTex films (x = 0 - 0.5)

Abstract In order to measure the complex conductivity of superconducting films much thinner than the penetration depth, for wide temperature range including the vicinity of T c, we developed a novel cavity pertubation technique utilizing microwave electric fields parallel to the film surface. By applying this newly developed technique to FeSe1−x Te x films, we found changes in the temperature dependence of the superfluid density and the quasiparticle scattering rate at the nematic boundary, which is in agreement with the results already obtained in the measurement in microwave magnetic field at lower temperatures. These changes in the nematic boundary suggests a variation in the superconducting gap structure between samples in the nematic and non-nematic phases.

Graphite Content Identification with Laboratory and Field Spectral Induced Polarization Measurements

Graphite, a critical raw material, prompts interest in assessing former quarries for volumetric content, driving the need for accurate prospection techniques. We explore the efficacy of spectral induced polarization (SIP) imaging at field scale for this purpose. Field measurements in a quarry with unknown graphite content underscore the need for assessment before drilling due to abrupt topography. Due to the lack of ground truth required to calibrate existing petrophysical models, we propose using SIP laboratory measurements to achieve the quantitative interpretation of the imaging results. We conducted experiments at two scales: rock plugs for material response and ground rocks of varying sizes for textural analysis. The rock plugs allow us to investigate the response of the material, while the ground samples permit us to understand changes in the SIP response for varying textural properties. Our lab work establishes power-law relationships between polarization (expressed in terms of normalized chargeability) and graphite content, as well as relaxation time and grain size. Salinity dependence is noted between chargeability, normalized chargeability, and relaxation time. Utilizing these findings, we provide a quantitative interpretation of field SIP imaging results.

Evaluation of graphene oxide-coated sand adsorption on ammonium, lead and FA with molecular dynamic simulation and spectral induced polarization

Evaluation of graphene oxide-coated sand adsorption on ammonium, lead and FA with molecular dynamic simulation and spectral induced polarization

Induced polarization of clay-rich materials — Part 3: Partially saturated mixtures of clay and pyrite

Source rocks for oil and gas are often associated with shales that are rich in pyrite and kerogen. Induced polarization is a suitable tool to characterize these formations. We develop a new experimental database of 43 laboratory experiments using mixtures of clay and pyrite under fully or partially water-saturated conditions. The liquid water saturation is in the range of 20%–100%, whereas the pyrite content is in the range of 0%–17%. Spectral-induced polarization measurements are performed in the frequency range of 0.1 Hz to 45 kHz at room temperature (approximately 25°C ± 1°C). The complex conductivity spectra are fitted with a Cole-Cole model, and the Cole-Cole parameters are determined using a stochastic procedure based on a Markov chain Monte Carlo sampler. The Cole-Cole parameters associated with low-frequency dispersion are then plotted as a function of the (water) saturation and pyrite content (volume fractions). Four predictions of the model are tested against the experimental data. We find that the model is able to explain the results, including (1) the chargeability depends on the pyrite content in a predictable way, (2) the (instantaneous) conductivity depends on the saturation according to Archie’s law, (3) the Cole-Cole exponent does not depend on the saturation and pyrite content (except at very small pyrite content of <1 vol%), and (4) the relaxation time is inversely proportional to the instantaneous conductivity. We also develop a more advanced petrophysical model using a double Cole-Cole distribution, in which one distribution is associated with the clay minerals and the other is associated with the pyrite.

Changing Cr-Retention Capacity of Magnetite-Coated Sand Captured by Spectral Induced Polarization (SIP)

Changing Cr-Retention Capacity of Magnetite-Coated Sand Captured by Spectral Induced Polarization (SIP)

Correlation between induced polarization and sulfide content of rock samples obtained from seafloor hydrothermal mounds in the Okinawa Trough, Japan

AbstractThe physical properties of seafloor massive sulfides are crucial for interpreting sub-seafloor images from geophysical surveys, shedding light on the evolution of seafloor mineral deposits. While some studies have explored the relationship between electrical properties and the volume of conductive minerals in rocks from seafloor massive sulfide deposits, they primarily focused on artificial samples, leaving the characteristics of natural samples less understood. Moreover, there has been no comprehensive study detailing the general characteristics of electrical properties, particularly chargeability and relaxation time, in relation to the volumetric fraction of sulfides in rocks from massive sulfide mounds in typical hydrothermal areas. In this study, we employed complex conductivity measurements, elemental concentration analysis, and mineral content identification on to rock samples from the active hydrothermal zones of the Okinawa Trough in Japan. The complex conductivity observed was remarkably high, with a pronounced imaginary component and a broad frequency range. This is attributed to induced polarization extending beyond our measurement range. The rock samples were rich in conductive sulfide minerals such as pyrite, chalcopyrite, and galena. Using the Cole–Cole rock physics model, we established a correlation between rock chargeability and relaxation time coefficient with the volume fraction of conductive sulfide minerals, which deviated from previous findings. The intensity of induced polarization was notably higher than anticipated in earlier studies using artificial samples. Furthermore, we observed a distinct positive correlation between the coefficient of relaxation time and the increase in sulfide volume, likely due to the geometric characteristics of the sulfide minerals. Our findings suggest that rocks in massive sulfide mounds may generally construct sulfide clusters that lengthen the conductive path of the electrical carrier. Graphical Abstract

Monitoring of copper adsorption on biochar using spectral induced polarization method

Copper contaminant generated from mining and industrial smelting poses potential risks to human health. Biochar, as a low-energy and cost-effective biomaterial, holds value in Cu remediation. Spectral Induced Polarization (SIP) technique is employed in this study to monitor the Cu remediation processes of by biochar in column experiments. Cation exchange at low Cu2+ concentrations and surface complexation at high Cu2+ concentrations are identified as the major mechanisms for copper retention on biochar. The normalized chargeability (mn) from SIP signals linearly decreased (R2 = 0.776) with copper retention under 60 mg/L Cu influent; while mn linearly increases (R2 = 0.907, 0.852) under high 300 and 700 mg/L Cu influents. The characteristic polarizing unit sizes (primarily the pores adsorbing Cu2+) calculated from Schwartz equation match well with experimental results by mercury intrusion porosimetry (MIP). It is revealed that Cu2+ was driven to small pores (∼3 μm) given high concentration gradient (influent Cu2+ concentration of 700 mg/L). Comparing to activated carbon, biochar is identified as an ideal adsorbent for Cu remediation, given its high adsorption capacity, cost-effectiveness, carbon-sink ability, and high sensitivity to SIP responses – the latter facilitates its performance assessment.

Study on the Complex Electrical Response Characteristics of Loaded Coal and the Control Mechanism of the Main Fracture System Structure.

The structure of coal seam fractures is the main physical property of coalbed methane reservoir evaluation, and the complex resistivity method is a potential geophysical evaluation method for coal seam fractures. In this study, cylindrical coal samples with axial directions perpendicular to the bedding, face cleat, and butt cleat were prepared. The complex electrical parameters of the loaded specimens were tested with test frequencies ranging from 1 Hz to 10 kHz. The complex electrical response characteristics of the loaded coal are summarized, and the control mechanism of the main fracture system structure is analyzed. The results indicated that (1) as the loading pressure increased, the resistance R and the absolute values of reactance X(|X|) gradually decreased, especially in the frequency band where R slowly decreased and the characteristic frequency of X, the decreased amplitude was more significant, and the cutoff frequency of R and the characteristic frequency of X all gradually increased. (2) The complex electrical properties of coal show obvious anisotropic characteristics. Both R and |X| decreased sequentially according to the direction perpendicular to the bedding, face cleat, and butt cleat; the cutoff frequency of R and the characteristic frequency of X all increased sequentially. (3) The dispersion phenomenon of the complex electrical properties of coal is attributed to the induced polarization; the elevated loading stress enhances the polarization effects of the molecular-induced moments of the coal skeleton, and the anisotropic difference of the complex electrical properties is due to the difficulty in the degree of transport of charged particles induced by structural differences of the main fracture system. (4) The resistance R3 and capacitance Xc were selected as the complex electrically sensitive parameters of the loaded coal orthogonal fracture structures. A logarithmic inversion model reflecting the main fracture system structure of coal was constructed. This provides a certain theoretical basis for efficient electrical exploration of coal reservoir fracture structures.

Quantifying salinity in heterogeneous coastal aquifers through ERT and IP: Insights from laboratory and field investigations

The lithological and stratigraphical heterogeneity of coastal aquifers has a great influence on saltwater intrusion (SI). This makes it difficult to predict SI pathways and their persistence in time. In this context, electrical resistivity tomography (ERT) and induced polarization (IP) methods are receiving increasing attention regarding the discrimination between saltwater-bearing and clayey sediments. To simplify the interpretation of ERT data, it is commonly assumed that the bulk conductivity mostly depends on the conductivity of pore-filling fluids, while surface conductivity is generally disregarded in the spatial and temporal variability of the aquifers, particularly, once the aquifer is affected by the presence of saltwater. Quantifying salinities based on a simplified petrophysical relationship can lead to misinterpretation in aquifers constituted by clay-rich sediments. In this study, we rely on co-located data from drilled boreholes to formulate petrophysical relationships between bulk and fluid conductivity for clay-bearing and clay-free sediments. First, the sedimentary samples from the drilled wells were classified according to their particle size distribution and analyzed in the lab using spectral IP in controlled salinity conditions to derive their formation factors, surface conductivity, and normalized chargeability. Second, the deduced thresholds are applied on the field to distinguish clay-bearing sediments from brackish sandy sediments. The results are validated with logging data and direct salinity measurements on water samples. We applied the approach along the Luy River catchment and found that the formation factors and surface conductivity of the different unconsolidated sedimentary classifications vary from 4.0 to 8.9 for coarse-grained sand and clay-bearing mixtures, while normalized chargeability above 1.0 mS.m−1 indicates the presence of clay. The clay-bearing sediments are mostly distributed in discontinuous small lenses. The assumption of homogenous geological media is therefore leading to overestimating SI in the heterogeneous clay-bearing aquifers.

Electrical Responses of Modified Mineral Surfaces as Observed With Spectral Induced Polarization and Atomic Force Microscopy

AbstractAtomic force microscopy (AFM) and spectral induced polarization (SIP) are widely used to investigate the electrical properties of mineral surfaces at vastly different scales of measurement. We compare AFM and SIP measurements made on two different materials (glass beads and silica gel) subjected to etching, deposition of iron oxide particles, and inclusion of calcite grains. We found that the treatments produced qualitatively consistent behaviors in the AFM and SIP data. Direct AFM measurements of surface charge density for silica and calcite surfaces were quantitatively compared to values estimated from the SIP results using a grain polarization model. No statistically significant difference (at a 95% confidence level) was found between the surface charge density of silica estimated by AFM (2.3 ± 6.6 mC/m2 for glass beads and 1.6 ± 0.1 mC/m2 for silica gel) versus SIP (5.4 ± 4.4 mC/m2 for glass beads and 1.6 ± 0.5 mC/m2 for silica gel). The surface charge density for calcite determined by AFM (43.5 ± 12.9 mC/m2) was approximately 19 times higher than that found for silica. While the charge density of calcite surfaces determined by SIP was also generally higher than that found for silica, different treatments produced significantly different values between 4.7 and 258 mC/m2 (with a maximum 95% CI of ±8.7 mC/m2). Several possible explanations exist for the range of the observed SIP measurements, including aging of the calcite surfaces. Overall, this study suggests the potential for the complementary use of AFM and SIP measurements to constrain future investigations of polarization mechanisms in porous media.

The Fundamental Flaws of the Waxman-Smits and Dual Water Formulations, Attempted Remedies, and New Revelations from Historical Laboratory Complex Conductivity Measurements

The Waxman-Smits formula was introduced in 1968 as a parallel conductance model to improve previous models. A careful inspection of the Waxman and Smits model reveals it is not a parallel conduction model by the conventional definition. First, Waxman-Smits assumed that “the electrical current transported by the counterions associated with the clay travels along the same tortuous path as the current attributed to the ions in the pore water” (Waxman and Smits, 1968), removing an essential feature of a parallel conduction model, that there be two separate conductors. Based on this assumption, they assign the same geometrical factor to both current paths. The geometrical factor is defined as the reciprocal of the formation resistivity factor (1/F or σm). Waxman-Smits found experimentally that a shaly sand appeared to have an F that was larger than a clean sand and introduced F* to account for this. Therefore, the tortuosity of the current paths through the clay and the pore water were deemed to be equivalent, with both tortuosities increasing equally as the clay content increased. Second, a parallel model requires the bulk conductivity of a volume to be weighted by the fractional volumes of the separate clay and interstitial water current paths. Clavier et al. (1977) discovered during the field testing of the new 1.1-GHz electromagnetic propagation tool that there existed a volume of clay water of near-constant salinity in shales. These two concepts are not accounted for in the Waxman-Smits model. A re-evaluation of the Waxman-Smits database by Clavier et al. (1977, 1984) revealed the F* increase was primarily due to the Waxman-Smits model not accounting for the physical presence of the volume of the clay water. The inclusion of the clay water volume in the dual water model produces a true parallel conductivity model. However, like Waxman-Smits, it assigns the same tortuosity to both the clay and pore water current paths. This assumption seems dubious based on observations of scanning electron microscope (SEM) photos showing actual clay morphologies. Laboratory measurements of pure clay and glass beads would allow one to quantify tortuosity changes due to the introduction of clay into an otherwise pure glass bead environment. Theoretically and experimentally, the value of the clay water conductivity (Ccw) at room temperature was found to be 6.8 S/m. Therefore, for a pore water conductivity (Cw) less than 6.8, the clay adds to the rock conductivity relative to an Archie rock, as written in the Waxman-Smits model. However, when Cw is greater than 6.8, the clay water subtracts from the rock conductivity relative to an Archie rock. This cannot be accommodated by the Waxman-Smits formulation. To correct for this model deficiency, B was made a function of salinity and temperature when, theoretically, it is a function of temperature only. Thirdly, neither model accurately predicts the rock conductivity at pore water salinities below approximately 0.5 to 1 S/m. Having a proper model at these lower salinities is important for geothermal evaluations, waterflooded reservoirs, and naturally occurring freshwater reservoirs. We propose a correction method based on our knowledge gained from the study of the quadrature conductivity measurement from cores and recent laboratory measurements.

The effect of heterogeneous contact impedances on complex resistivity measurements

SUMMARY Spectral induced polarization (SIP) and electrical impedance tomography (EIT) are advanced approaches for structural characterisation of soils and rocks based on measurements of the frequency-dependent complex electrical resistivity. Accurate SIP and EIT measurements require the reduction of electrode effects, which have often been attributed to the polarization of metal electrodes in the current pathway. In this study, we demonstrate an alternative electrode effect that influences the accuracy of complex impedance measurements. This alternative electrode effect is caused by a heterogeneous distribution of the contact impedance, which can occur due to ageing or corrosion of the electrode. First, theoretical considerations are given to explain how this electrode effect influences the accuracy of complex impedance measurements. Next, we demonstrate using SIP and EIT measurements that this alternative mechanism better explains the observed electrode effects than previous explanations based only on the presence of metal. The SIP measurements were made on a water-filled container with a metal sphere between two non-polarizing electrodes, which showed that the mere presence of metal in the current pathway only leads to small electrode effects that are typically well below 1 mrad even for small electrode spacings. When the same metal sphere was used as a potential electrode, the observed electrode effect was about 2.5 times larger and varied with rotation. These changes can be explained by a heterogeneous contact impedance resulting in a varying electrode voltage. The EIT measurements were performed with a cylindrical sample holder filled with water. They are used to illustrate how the presented theoretical considerations can be used to investigate the magnitude of the electrode effects for different electrode configurations. We conclude that electrode effects are important to consider for laboratory SIP and EIT measurements and field measurement with closely spaced electrodes.

Organic Matter Matters—The Imaginary Conductivity of Sediments Rich in Solid Organic Carbon

AbstractSolid organic matter (OM) is a biogeochemically relevant constituent of soils and sediments. It also affects sediments' geophysical properties, but is often overlooked in hydro‐ and biogeophysical approaches for the characterization of the shallow subsurface. Here, we explore the potential of spectral induced polarization (SIP) to delineate OM‐rich zones in the subsurface and provide insights into the mechanisms that drive OM‐polarization using measurements on both field cores and artificial OM‐sand mixtures. Both, field samples and artificial mixtures showed a linear relationship between the total organic carbon (TOC) content and charge storage (imaginary conductivity). The high cation exchange capacity of OM drives the increase in polarization and can help in delineating potentially microbially active OM‐rich zones in cores or field surveys. To avoid misinterpretation of SIP data in unconsolidated media, we strongly suggest quantifying TOC content in sediment samples to accompany the interpretation of field surveys.